Talc has been used extensively as a sealing material in the bore of ceramic spark plug insulators, as well as between the exterior of spark plug insulators and metal shells in which the insulator assemblies are supported. Since talc is an electrically non-conducting material, it must be packed around an electrical conductor when it is used to seal the bore of an insulator. In such service, it performs the dual function of preventing gas leakage through the insulator bore, and of locking the electrode which extends through the bore so that the electrode is not caused by the pressure to which it is subjected under service conditions to move longitudinally of the bore away from a cylinder within which it is installed. Talc alone, suggested, for example, as a sealing material of the type in question by U.S. Pat. Nos. 2,020,967 and 2,367,445, shows gradually decreased holding power under service conditions which subject it to temperatures in excess of about 900.degree. F., as well as a significantly decreased ability to prevent gas leakage when subjected to temperatures in excess of about 1300.degree. F.
Several spark plugs have also been found, believed to have been made by a German company, which had a seal in the upper part of the insulator bore containing talc, aluminum metal powder and silicon metal powder. The seal in one such plug contained 41 percent* talc, 40 percent aluminum metal powder and 19 percent silicon metal powder. To date, no patent or other publication disclosing this spark plug has been found. A seal made from talc, aluminum metal powder and silicon metal powder in the stated proportions has been produced, and has been tested. Such a seal has been found to be substantially equivalent to a seal made of talc, alone. It can be used satisfactorily to anchor a center electrode in the upper part of the bore of a spark plug insulator, and to prevent gas leakage when so positioned. The leakage of gas through such seals, however, increases catastrophically when the seals are operated under such service conditions that they reach a temperature of 1220.degree. F. or higher, for example in the lower portion of an insulator bore; furthermore, at temperatures above about 1220.degree. F. the aluminum-containing seal is virtually ineffective at anchoring the center electrode and the seal composed of talc alone exhibits a somewhat decreased effectiveness in this respect. U.S. Pat. No. 2,437,205 also suggests the use of either talc or a mixture of talc and comminuted metal to form such a seal, but does not indicate that the mixture has any advantage over talc. FNT *The terms "percent" and "parts" are used herein, and in the appended claims, to refer to percent and parts by weight, unless otherwise indicated.